Long before Wegener introduced his ideas of continental drift, geologists had been faced with two other great mysteries of the ocean. First, they wondered what happened to all of the sediment that was carried into the sea by the rivers that emptied into the ocean. Scientists had observed how ponds and lakes would eventually fill in with silt and sand carried in by the streams that flowed into them. If Hutton and Lyell were right about the age of Earth and the fact that processes like erosion and deposition of sediment have been going on for millions of years, then the oceans should be almost filled with sediment. When scientists were finally able to get a close-up look at the seafloor, they found that some sediment covered the bottom, but not nearly enough to account for all the miles of rock that had been eroded off the continents.
The second mystery that puzzled the scientific community was the fact that the ocean contained almost no fossils of creatures that were greater than about 200 million years old. On the continents, however, there were fossils that were three times older and rocks that were close to 4 billion years old. It appeared that the seafloor was much younger than the rest of the planet. If the ocean basins and continents had been in the same positions since Earth first formed, then the rocks from each should be the same age.
Based on his ongoing research, and the maps developed by Heezen and Tharp, Harry Hess thought he knew the answer. He offered a bold new theory to explain it, an idea that has become known as the theory of seafloor spreading. Hess suggested that the ocean floor is like a giant conveyor belt. At the mid-ocean ridges, active volcanoes pump out lava that forms new crust. As the crust is added, the two sides of the seafloor push apart and the ocean gets wider. Along the edges of the continents where the submarine trenches are found, the ocean floor is being consumed. Hess proposed that the trenches are like rips in the surface of the Earth where the old
Plate Tectonics Model
ocean floor is being forced back down into the Earth by a process called subduction. Because the rock that makes up the ocean floor is usually denser than the rocks that make up the continents, the ocean crust will naturally sink underneath the continents.
As the crust sinks, the sediment that was riding on top of the seafloor would get scraped off at these subduction zones and plastered onto the edge of the continent, forming mountains along the edge of the coast.
Hess first presented this idea in 1959 but did not formally publish his theory until 1962 in a report entitled History of Ocean Basins. The concept was brilliant, and it tied up many geological lose ends. It explained why the rocks and fossils in the ocean were so much younger than the rocks found on the continents. It also explained what happened to all the missing sediment. Most importantly, it gave new life to the idea of continental drift. Seafloor spreading, driven by the convection currents that Arthur Holmes had suggested could be found deep in the Earth, would provide the means to make the continents move. But was there any hard evidence to prove that seafloor spreading existed? It turns out the evi dence was already there, frozen in the rocks, and just waiting to be discovered.
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